Several lines of evidence suggest that deficits in glutamatergic function may play a role in the pathophysiology of schizophrenia. Ketamine is an FDA approved anesthetic agent. It is a specific noncompetitive antagonist of the NMDA-type glutamate receptor and is currently being used by several research groups in subanesthetic doses to probe glutamatergic function. The purpose of this study is to examine the effects of subanesthetic doses of ketamine on metabolic rate with FDG/PET, behavior, neuroendocrine parameters and neuropsychologic function in schizophrenic patients and healthy controls. To determine if neuroleptic treatment attenuates ketamine effects, patients will be tested during both neuroleptic-free and neuroleptic treatment conditions. Ketamine will be administered in a single-dose, double-blind placebo controlled paradigm.